1. Field of the Invention
This invention relates to instruments of the type using a battery and more particularly to the detection of the voltage level or remaining voltage of a battery and the detection of a minimum voltage at which such instrument is operating (hereinafter referred to as an under cut voltage V.sub.UCUT) by means of the same circuitry thereby ensuring high quality detection of the battery voltage level without any variation.
2. Description of the Prior Art
With conventional portable VTRs, two methods have been used to detect the voltage level of a battery, namely, a pointer meter indication method and another method which detects the voltage level by applying the battery voltage to a plurality of comparators having different reference voltages. The former pointer meter indication method requires, in addition to a circuit for detecting the under cut voltage, a separate battery voltage level detecting circuit, and two kinds of adjustments, i.e., the adjustment of the pointer meter indication and the adjustment for under cut voltage detection are necessary in consideration of variations in the characteristics of component parts to be used, thereby increasing the cost of the apparatus.
FIG. 1 of the accompanying drawings shows an example of the latter prior art method using a plurality of comparators having different reference voltages. In FIG. 1, a reference voltage generating circuit 22 generates a given reference voltage V.sub.ref which is lower than the under cut voltage V.sub.UCUT from the battery voltage V.sub.BATT generated from a battery 21 and it comprises a resistor and a Zener diode whose forward voltage is lower than the under cut voltage. Reference voltages V.sub.a, V.sub.b, V.sub.c and V.sub.d of different voltage values are produced by resistor division or the like from the reference voltage V.sub.ref generated from the reference voltage generating circuit 22 and they are applied to comparators 24, 25, 26 and 27, respectively. It is assumed that there is a relation V.sub.a &gt;V.sub.b &gt;V.sub.c &gt;V.sub.d. A voltage V.sub.e generated from a voltage generating circuit 23 is applied to the other terminal of the comparators 24, 25, 26 and 27, respectively. The voltage V.sub.e varies with variation of the battery voltage V.sub.BATT and it comprises for example a variable resistor VR.sub.2 as shown in the Figure. The variable resistor VR.sub.2 is adjusted so that the voltage V.sub.e becomes equal to the voltage V.sub.d when the battery voltage V.sub.BATT is equal to the under cut voltage V.sub.UCUT. With this construction, the voltage V.sub.e varying with variation of the battery voltage is compared with the fixed voltages V.sub.a, V.sub.b, V.sub.c and V.sub.d which are different in voltage values from one another in the comparators 24, 25, 26 and 27 and the voltage level of the battery is displayed on a display device 29 through a display circuit 28 in accordance with the comparison result. On the other hand, when the output of the comparator 27 goes to a low level, it is an indication that the voltage V.sub.e is lower than the voltage V.sub.d and hence the battery voltage is lower than the under cut voltage, thereby applying a command to a mechanism control circuit 30 to disable the operation of the instrument. With this prior art method, it is necessary to use the same number of comparators as the number of resolutions of the battery voltage level and the cost of the construction is increased very greatly. Another disadvantage is that in order to effect detection with a high degree of accuracy, the voltages V.sub.a, V.sub.b and V.sub.c must be adjusted and thus the construction requires a large number of manhours.